Why is my iPhone data transfer taking so long?

I am trying setup a new iPhone15 and transferring the data from my existing iPhone11 by keeping them nearby. While the ios18 got installed, the process of data transfer is taking forever...It worked for more than 10 hours and finally my old phone stopped sharing and the new one got hung on the screen "Software Update". This same thing happened second time - during the first time I had to restart the new phone and had to start the process all over...

anyone faced similar situation?

[Re-Titled by Moderator]